As far as I can tell, the reorganisation is at a very early stage, councils are still arguing about who wants which bits, and what they'll do with all the debts.
There would be a good case if everything was agreed, and that this council was going to merge with that one in a year. I'm pretty sure that has been the situation when elections have been cancelled before for this reason. But that's not where we are, we're at the opening stages, this haggling could go on for years.
I have a suspicion that the Labour party's unpopularity, from very soon after they entered government, was the main reason for pushing the reorganisation, so that they'd have an excuse for cancelling elections. It's all a bit "dog ate my homework".